1. Field of the Invention
The present invention relates to a method of forming a high quality, high melting-point metal film on a substrate surface, by means of chemical vapor deposition (CVD).
2. Description of the Prior Art
Recently, a thin, high melting-point metal film has come to be used as a fine metallic wiring material for high-density integrated circuits. The group of high melting-point metals includes Ti, W, Mo, and Ta. While the following description exemplifies tungsten, it can apply equally to the other high melting-point metals.
In order to form a thin W film, a gaseous phase growth by use of CVD is employed, along with, for example, WF.sub.6 and H.sub.2 being used as the raw material gases. The gaseous phase reaction involved in this technique is as follows: EQU WF.sub.6 +3H.sub.2 .fwdarw.W+6HF
It is important to note that in the gaseous phase reaction outlined above, the W thin film is unlikely to grow on an insulation film, for example, a SiO.sub.2 film, because the insulation film is believed not to provide a suitable catalytic surface for promoting the gaseous phase reaction, unlike a metal or semiconductor surface which provides an appropriate catalytic surface therefor. It follows that it is possible to selectively form a W thin film on only predetermined regions of a substrate surface, thus making it possible to form a fine W wiring pattern without the need to employ a patterning step. This particular technique is known as a Selective CVD Method.
However, the formation of a thin W film, by use of the CVD described above, gives rise to a serious problem. Specifically, the selectivity of the W film formation is not complete In particular, where the gaseous phase growth is allowed to continue for a long time, in a reaction temperature (or substrate temperature) exceeding 400.degree. C., W becomes deposited, to a slight extent, on the insulation film itself. In particular, where the insulation film is formed of, for example, SiO.sub.2, HF, formed by the gaseous phase reaction carried out in high temperature conditions, is believed to react with SiO.sub.2, to form SiF.sub.4 on which W is likely to be deposited, and to impair the selectivity of the W film formation, this reaction being as follows: EQU SiO.sub.2 +4HF.fwdarw.SiF.sub.4 +2H.sub.2 O
However, where a wiring layer of a W thin film is formed on a semiconductor substrate by, for example, a selective CVD method, the treatment time for the individual wafers is short. Thus, reduction in selectivity for individual wafers is relatively small, because each wafer is replaced by new wafer.
On the other hand, a CVD apparatus is used repeatedly over a long period of time. Thus, in the case of, for example, a diffusion furnace type CVD apparatus, the deposition of W on the inner wall of a quartz (SiO.sub.2) reaction tube continues over an extended period of time. To be more specific, when the CVD operation is performed over a long period of time, fine W particles become deposited primarily on the inner wall of the reaction tube. What should be noted is that the deposited fine W particles provide the catalytic surface for the W growth reaction referred to previously, thereby leading to a rapid W deposition thereafter. The rate of growth of the W film, on the semiconductor surface, lowers rapidly in accordance with the amount of W deposition on the inner wall of the reaction tube. It follows that it is impossible to form a W film rapidly in high-temperature conditions. In addition, it is impossible to control the thickness of the W film. An additional difficulty arises, in that the W particles deposited on the inner wall of the reaction tube are only weakly attached to the surface thereof, with the result that they tend to peel off the surface of the wall during the vapor phase growing step. The W particles in question drop onto the substrate, with the result that particle-like defects appear on the film growing on the substrate. Naturally, these defects present serious problems to be solved, in order to form a fine circuit in a high density integrated circuit.
The difficulties noted above occur not only in the diffusion type CVD apparatus but also in a stainless steel CVD apparatus called a cold wall type. It should be noted that W particles are likely to become deposited on the surface of the stainless steel itself. Thus, in the cold wall type CVD apparatus, the wall of the reaction chamber is cooled so as to prevent the deposition of W on the wall. However, a susceptor having a substrate mounted thereon is provided with a heater, to heat the substrate to a temperature at which the vapor phase growth reaction can take place, with the result that W particles are deposited on the susceptor surface. In another type of cold wall CVD apparatus, the substrate mounted on the susceptor is heated by irradiation of infrared rays through a window of the reaction chamber. In this case, the susceptor and inner wall of the reaction chamber are also partly irradiated with the infrared rays. The irradiated parts become heated, thereby to cause the deposition of W. It follows that the difficulties, such as a marked reduction in the W film growth rate, which are inherent in the diffusion furnace type CVD apparatus, also take place in the cold wall type CVD apparatus.